Purcell had the insight that in a cavity, the number of states available for photons is not quadratic in frequency anymore. Instead, a cavity on resonance has a photon density of states that is proportional to the “quality factor”, Q, of the cavity, and inversely proportional to the size of the cavity. The better the cavity and the smaller the cavity, the higher the density of states at the cavity resonance frequency, and off-resonance the photon density of states approaches zero. This means that the spontaneous emission rate of atoms, a property that seems like it should be fundamental, can actually be tuned by the local environment of the radiating system. The Purcell factor is the ratio of the spontaneous emission rate with the cavity to that in free space. While I was doing some writing today, I decided to look up the original citation for this idea. Remarkably, the “paper” turned out to be just an abstract!

The Oatmeal is a fantastic comic that I recommend that you make a habit of reading. However, even the greatest can go astray, and I’m pained to admit that The Oatmeal has done so regarding someone I regard very highly, and that’s Nikola Tesla. Alas, The Oatmeal has fallen prey to Tesla idolatry, confusing his genius for godhood and of course, setting up the now all-too-common ‘Edison as Tesla’s arch-villain’ narrative. There are quite a few errors and misconceptions about both Tesla and Edison in this comic. But they’re errors that I’ve seen before and are often repeated, so it’s worth the time, I think, to address some of the big ones.

Books

You've read the blog, now try the books:

Eureka: Discovering Your Inner Scientist will be published in December 2014 by Basic Books. "This fun, diverse, and accessible look at how science works will convert even the biggest science phobe." --Publishers Weekly (starred review) "In writing that is welcoming but not overly bouncy, persuasive in a careful way but also enticing, Orzel reveals the “process of looking at the world, figuring out how things work, testing that knowledge, and sharing it with others.”...With an easy hand, Orzel ties together card games with communicating in the laboratory; playing sports and learning how to test and refine; the details of some hard science—Rutherford’s gold foil, Cavendish’s lamps and magnets—and entertaining stories that disclose the process that leads from observation to colorful narrative." --Kirkus ReviewsGoogle+

How to Teach Relativity to Your Dog is published by Basic Books. "“Unlike quantum physics, which remains bizarre even to experts, much of relativity makes sense. Thus, Einstein’s special relativity merely states that the laws of physics and the speed of light are identical for all observers in smooth motion. This sounds trivial but leads to weird if delightfully comprehensible phenomena, provided someone like Orzel delivers a clear explanation of why.” --Kirkus Reviews "Bravo to both man and dog." The New York Times.

How to Teach Physics to Your Dog is published by Scribner. "It's hard to imagine a better way for the mathematically and scientifically challenged, in particular, to grasp basic quantum physics." -- Booklist "Chad Orzel's How to Teach Physics to Your Dog is an absolutely delightful book on many axes: first, its subject matter, quantum physics, is arguably the most mind-bending scientific subject we have; second, the device of the book -- a quantum physicist, Orzel, explains quantum physics to Emmy, his cheeky German shepherd -- is a hoot, and has the singular advantage of making the mind-bending a little less traumatic when the going gets tough (quantum physics has a certain irreducible complexity that precludes an easy understanding of its implications); finally, third, it is extremely well-written, combining a scientist's rigor and accuracy with a natural raconteur's storytelling skill." -- BoingBoing